This invention relates in a totally general manner to the manufacture of ceramic tiles.
More specifically it relates to the forming of large-format tiles, hereinafter called slabs, having sides well exceeding one meter, for example 1.5xc3x972 m.
As is well known, modern ceramic technology has developed to the point of constructing hydraulic presses of very high power, such as to enable said large-dimension ceramic slabs to be obtained starting from at least one powder material which is substantially dry, i.e. having a moisture content between 1 and 11%
A typical use for such slabs, either whole or cut into pieces to provide a sort of mosaic, possibly in combination with particular surface treatment or decoration, is in facing large interior and exterior surfaces, for example the interior walls of a stadium or the exterior walls of a large building.
Said large-dimension slabs are prepared by moulds of traditional type, i.e. consisting of a unit comprising a lower plate positioned on the bed of the press, and provided upperly with a die, usually a reverse face die; an intermediate plate provided with a forming cavity within which said reverse face die is slidingly received, said intermediate plate being fixed or movable; and an upper plate carried by the vertically movable crosspiece of the press and provided lowerly with a die, usually an exposed face die, which cooperates with the reverse face die to form the slab.
It should be noted that between the constituent elements of the mould there are provided openings or clearances through which the air present in the powders to be pressed is discharged, these openings or clearances having relatively very small dimensions, much smaller than the powder particle size. The reason for stating this will become apparent hereinafter. Said forming cavity is loaded with a soft mass of at least one ceramic material in powder form which by the nature of things also contains air, this air having a volume which is substantially of the same order of magnitude as that of the powder alone, or in other words substantially one half that of the overall soft mass.
The problem therefore arises of evacuating said air during pressing to prevent the slab undergoing damage during firing, such as surface separation, deterioration, cracking or even breakage, due to the air mass trapped within the slab interior.
This problem is important seeing that the larger the slab format, the greater the quantity of air to be evacuated, of the order to 60 liters in the case of a 1.5xc3x972 m slab of 2 cm thickness, and the longer the path which this air has to take to reach the periphery of the mould cavity.
To extract the air from the initial powder material it is usual to maintain the mould in its closed configuration, i.e. with the dies clamped against the powder layer with the force necessary for the process underway, for the time required for the pressurized air to migrate from the interstices between the powder particles to the periphery of the mould cavity, where it escapes to the outside at high speed through said small openings or clearances in the mould.
However the time required to evacuate the air, usually known as deaeration, is in sharp contrast to modern operating cycles, which are characterised by massive production rates.
Said deaeration problem is a common problem in the forming of the various types of slabs, which are all included within the following three basic types: support or biscuit slabs, i.e. consisting of at least one not particularly valuable powder material, which are intended to be glazed separately before or after firing; fine porcellainized stone slabs, which do not need to be glazed as the desired aesthetic effect is provided by the actual powder material used to form at least the upper part of the slab body; and so-called pressure-glazed slabs which after pressing present a lower layer of relatively inexpensive material such as atomized clay, and an upper layer of material having the desired aesthetic characteristics, typically powdered ceramic glaze.
To produce these pressure-glazed slabs plants are known comprising essentially two moulds of traditional type connected together by a conveying system with which a decorating station, typically of dry type, is associated.
Base material is loaded into the first mould to form the body of the slab, after which this is pressed to obtain virtually total deaeration.
Having done this the blank obtained is then removed from the first mould, transferred to the at least one decorating station, then fed to the second mould where deaeration and compaction of the base material is completed, the glaze is deaerated and compacted, and the two are bonded together.
The operating pressure of the first stage is less than that of the second, for example a typical specific pressure of the first mould is 50-150 bar, whereas a typical specific pressure of the second mould is 250-600 bar. In such pressure-glazing plants, in addition to this inconvenient slow down in production due to the deaeration, problems can arise while loading the blank or precompacted slab into the die plate of the second mould. In this respect, and if operating under optimal conditions, in particular with a base material having a moisture content substantially equal to that typical or specific for the process under way, during said loading the blank must be virtually exactly aligned with the forming cavity of the second mould, and have virtually the same plan dimensions.
If this is not so, when the lower die descends, the blank remains temporarily engaged with at least one upper edge of the forming cavity, to then fall onto the completely lowered lower die, either because of vibration or following the descent of the upper die, this normally resulting in breakage of the blank and hence its discard because the next (second) pressing is unable to restore the integrity of the blank, and especially the integrity of the decoration.
If during the course of the day the moisture content of the starting material varies, as often happens, the problems are aggravated.
In this respect, if the moisture content increases, the expansion of the blank unloaded from the first mould decreases, and vice versa if the moisture content decreases.
Dealing with large-format blanks, a variation of even just 1% in the moisture content of the starting material leads to a dimensional variation in the blank sides of about 1 mm per linear meter.
Consequently, with said upward or downward dimensional variations, the loading means provided with the plant, which are obviously sized at the standard dimensions for the work in progress, and which are difficult to adjust in view of the relationship between the magnitude of such variations and the dimensions of such slabs, are unable to align the blank and the forming cavity of the second mould with the necessary accuracy.
In particular, if the dimensional variation is an increase, and is not particularly large, there is an increased probability that because of the said misalignment the blank remains engaged with at least one upper edge of the forming cavity during the descent of the lower die.
If instead the upward variation is fairly large, even assuming perfect alignment between the blank and the forming cavity the blank is unable to follow the lower die during its lowering, but remains resting on the perimetral edge of the cavity, with consequent breakage when the upper die descends.
Moreover if the dimensional variation is a decrease, whether small or large, as already stated misalignment can occur between the blank and forming cavity, with consequent unwelcome engagement between the blank and the upper edges of the cavity during the lowering of the lower die.
Consequently in this sector there is a great need for means able to overcome the aforestated complex problem.
The main object of the present invention is precisely to satisfy this requirement within the context of a constructionally simple, rational, reliable, durable, low-cost solution with a production capacity in line with modern production cycles and practically without rejects.
This object is attained by a method and plant having the characteristics indicated in the claims.
The invention is based on a working method comprising two separate stages of compression, and which, by virtue of the teachings of the invention, can be applied to the main or basic types of product specified in the introduction, i.e. biscuit or support slabs, fine porcellainized stone slabs, and pressure-glazed slabs.
In a totally general sense, to attain said object and in accordance with the proposed method, the final compacting of the large-dimension ceramic slab or tile is done in the total absence of lateral or peripheral retention action thereon.
All the objects of the invention are attained by the aforesaid method.
In this respect, said absence of lateral retention eliminates all problems associated with perfect centering of the blank being transferred from the first to the second pressing unit, and with the deaeration of the blank during final compacting, by which means the process is considerably accelerated and simplified.
Specifically, the second pressing unit of the invention essentially comprises a system of anvil and hammer type, described in detail hereinafter, which gives the overall plant the required characteristics of simplicity, low cost, low weight, reliability with time, a production rate of the same order of magnitude as that required for forming tiles of average format, and virtually total absence of rejects.
The characteristics and constructional merits of the invention will be apparent from the ensuing detailed description, given with reference to the figures of the accompanying drawings, which illustrate by way of non-limiting example three preferred embodiments of a plant for implementing the method of the invention.